A spokesman for Mr Ban, Martin Nesirky, says senior Iranian officials had assured Mr Ban that Iran understood the terms of his invitation.

Senior US officials said earlier the invitation must be withdrawn unless Iran fully and publicly endorses the formation of a transitional government for Syria that would pave the way for democratic elections.

Syria's main Western-backed opposition group says Iran must commit publicly by 1900 GMT to withdraw its "troops and militias" from Syria and abide by those terms, or the UN should withdraw the invitation.

"The statement today in Tehran by the foreign ministry spokesperson fell short by some measure of what the secretary-general expected to hear," Mr Nesirky said, adding that the UN has been in close contact with the US and Russians over the weekend.

He also denied Mr Ban's invitation caught the US by surprise, saying they were fully aware of the timing of the announcement.